1. Field of the Invention
The present invention relates to a vertical synchronizing signal detecting method to accurately detect the vertical synchronizing signal from a complex synchronizing signal.
2. Description of the Prior Art
In the art, in order to detect a vertical synchronizing signal from a complex synchronizing signal included in, for example, a complex video signal or the like, the complex synchronizing signal is fed to an integrating circuit with a predetermined time constant which then separates at its output side only a vertical synchronizing signal having a large pulse width as an output wave with a large voltage value. If the vertical synchronizing signal is used under such a condition that it is not completely separated from the horizontal synchronizing signal, the interval between the vertical synchronizing signal and the horizontal synchronizing signal immediately before the vertical synchronizing signal is different between first and second fields, and hence the interlace scanning becomes in complete due to the influence of the horizontal synchronizing signal. Therefore, in general it is necessary that integrating circuits are provided at 2, 3 and so on stages to remove the horizontal synchronizing signal completely.
In the system in which a digital signal such as a PCM (pulse code modulation) signal of an audio signal or the like is transmitted on a television signal in place of an ordinary video signal, it is necessary that high accuracy is required in time and also the vertical synchronizing is not erroneously detected due to drop-out or the like. The above requirement can not be satisfied by the above integrating method.
With the employment of the above PCM signal transmission system, the signal processing system is changed over in accordance with the difference of the television system such as the NTSC system, PAL system and SECOM system used in respective countries. When the above system is provided on several or signal IC (integrated circuit), it is preferred that the system is automatically changed over in accordance with the television system. In such a case, the difference of the television system can be detected by the difference of the periods of the vertical synchronizing signals (the period is 3 H, H being the horizontal period, in the NTSC system and 2.5 H in the PAL system), but in the case of the above integrating method, the above detection may be erroneously carried out due to the drop-out or the like and also the generation of jitters caused by the use of the integrating circuit proposes further problems.